As with any other mobile terminal, there is a desire to increase the battery-life of mobile terminals operating according to a continuous time communication scheme, such as Wideband Code Division Multiple Access (WCDMA). One method of increasing the battery-life is to decrease current consumption of the transmitter during low power operation by decreasing the bias current. Changing the bias current of the transmitter also changes the gain, and thus the power step, of the transmitter. In order to achieve a target output power, it is desirable to have a power control system for correcting the gain of the transmitter after a change in bias current.
Traditional power control systems use a closed loop architecture where the output of the transmitter is coupled to a power detector. In order to measure the gain, the power detector uses an averaging filter that requires approximately 200 microseconds to remove the amplitude modulation from the detected signal. However, unlike the Global System for Mobile Communications (GSM), WCDMA requires that mobile terminals continuously transmit in each transmit slot. Further, for WCDMA, any gain adjustments must be completed within approximately the first 50 microseconds of each time slot. Thus, the traditional power control systems which require approximately 200 microseconds to remove the amplitude modulation from the detected signal are not suitable for measuring and adjusting the gain of the WCDMA transmitter during the first 50 microseconds of a time slot such that the output power is essentially equal to the target output power. Further, typical power control systems may be used to detect the gain in one time slot and adjust the gain in the next time slot. However, the bias current may change from one time slot to the next. As such, correcting the gain for one time slot based on the gain during the previous time slot is not desirable.
There remains a need for a fast and accurate power control system for detecting and correcting the gain of a continuous time transmitter of a mobile terminal during an acceptable period at the beginning of a transmit slot.